• The ionosphere's inhomogeneities and turbulence has as a consequence a random changes in the amplitude and phase of the propagating wave. Referring to the frequency, the absorption due to the atmosphere in the range of 2.45-3 GHz is 0.05 dB, but this value increases sharply up to frequencies around 30 GHz. However, there is a “radiowindow” in the range 35-38 GHz (millimeter waves) in which the absorption decreases up to 0.4 dB. The other phenomenon to be considered is the electromagnetic dispersion due to the presence of rain, cloudiness etc... At 2.5-3 GHz the loss of energy due to the precipitation are estimated around 2-6%. At 35-38 GHz those losses are 8-11%. In this range a very strong loss (up to 12 dB) occurs in presence of dense cumulus-rain clouds and heavy rain. The total atmospheric attenuation of a 35 GHz beam in traversing vertically trhough the atmosphere in clear weather is 0.2 dB. Therefore the range 35-38 GHz can be used but only if the receiving ground antenna is placed in the earth's regions with small quantity of rain and cloudiness. But the factor which hamper the practical use of the millimetric waves is in the limited avalaibility of powerful and efficient microwave sources in this range [Koert et al,1991]. For instance, obtaining hundreds of kilowatts of power with a single source at millimeter wavelengths was impossible before the introduction of a particular class of microwave vacuum tubes: gyrotrons. They are capable of generating 200 kilowatts CW at 35 GHz. An other technical problem is connected with the necessity of phased arrays which are not available so far at 35-38 GHz. The choice of the site of the rectenna is very critical, since the weather condition can strongly effect the system performances. Usually, the rectennas are located in remote places. A site can be on the equator in a very dry area. The idea of placing a rectenna in Antartica presents some problems at 35 GHz: while a surface covered by snow is completely transparent in S-band, it is not at all visible at frequencies above 10 GHz as shown in Figure 7.28. Figure 7.26 Atmospheric Attenuation Across the Radio Microwave and Millimeter Regions of the Electromagnetic Spectrum [Elachi, 1987]
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